As yet, never before hacked sat nav.

Hi all, I doubt anyone can help but I'll try to explain the situation in as few words as possible yet still remain understandable.

Smart (yes, the silly small cars) had a Bosch touch screen sat nav stereo (called the Highline) from late 2010 until 2014. It was an over-priced and under-specced disgrace.
Because it's not a widely owned stereo, no one has attempted any hacking, except for me. However, every time I find an avenue to discover, something else stops me.

The main problem with the stereo is the sat nav program. After gaining access to the directory structure and trawling through files looking for English words, I found out that the software is Elektrobit's Street Director, the map files are .PSF (anybody know WTF these are or where to get updated ones?)
The issue with the software is that it doesn't allow full postcode search and the maps are all hideously out of date even when the car was brand new. (2 years out of date when I got mine).
If you ask to buy a new map disc, chances are they don't know how to order it but some have been quoted £150+ but you still don't get full post code search.

The unit runs WinCE6.0 and has 4GB of storage. The storage is annoyingly partitioned into 2. The 1st has the navigation software and the 2nd has the maps.

My 1st idea was to break straight into Windows when it starts and run things from there. Problem with that is there is no sound.
So then I tried copying the new sat nav program over into the storage and replace the original .exe with a renamed one for a different nav program.
The stereo starts as normal, loads up the standard menus, you click on navigation and it just sits there. This is because the standard menu application is "always on top" so anything you load is hidden behind the menu so you can see it or use it.

So now I'm looking at trying to make a replacement nav program "always on top" and hope it sits over the original menu.
Or... and I don't know if this is even possible... replacing all of the original software with software from one of those Chinese WinCE stereos or similar.

At least if the menu system was playing ball it might make things a bit easier to hack.

Any thoughts or ideas would be appreciated. I have tried and failed on a load of different ideas regarding this but have only mentioned stuff that worked or I think is important.

Cheers.